For qualitative and quantitative testing of liquid samples, particularly biological fluids from human beings and other animals, typical testing methods include chemical and biochemical techniques. These testing methods are largely performed using substrate-based lest elements which provide relatively fast results. Such testing methods are commonly performed in specialized laboratories, but are also commonly performed outside the laboratory, such as by an individual consumer. Testing methods using the substrate-based test elements typically rely on specially developed dry-chemistry processes often involving complex reactions using sensitive reagents. Nevertheless, such methods are simple enough to perform that non-experts can perform them easily.
Test elements of this kind typically comprise reagents embedded in one or more test layers. A reaction sequence is initiated by bringing a test element into contact with a liquid sample.
Typical reaction sequences between the dry reagent and the liquid sample lead to a measurable change on the test element, which change is characteristic for purposes of the particular analysis, such as measuring the concentration of an analyte of interest in the liquid sample. The change is typically evaluated visually or by means of an evaluation device corresponding to the test element. Reflection photometry is one example of a kind of evaluation technique in this regard. For purposes of this disclosure, test elements may also be referred to as analysis elements.
Several different types of test elements are known, typically differentiated by the measuring principle (e.g. optical or electrochemical) and the reagents provided on the test element for performing the analysis. Other differentiations include test element structure, such as the arrangement and fabrication of the layers forming the test element. For example, strip shaped test elements (test strips) are common and typically comprise an oblong plastic strip and at least one test layer affixed to the strip. Another example of a test element is a plastic frame enframing at least one test layer.
One typical substrate-based test element known in the art is used for determining blood glucose concentration for diabetics. Another example of strip-shaped diagnostic test elements is for urine analysis, e.g. one-field or multi-field test strips and/or test papers. Test elements that are evaluated with an evaluation device configured for electrochemical or optical analysis techniques are well known in the art, and persons of ordinary skill in the art will be familiar with several various embodiments of test elements and corresponding evaluation devices.
Various environmental factors can have a negative impact on the analytic functionality of test elements. For example, light, temperature and/or humidity are factors that can damage a test element such that faulty measurement results are provided. Obviously this can have dangerous consequences for any diagnosis based upon such results, or it can render the test elements unusable.
As an example in the context of test elements configured for optical evaluation, light-sensitive, temperature-sensitive, and/or humidity-sensitive compounds are used for which the dry color, even before contacting a liquid sample, can change due to the environmental factors in such an extreme way that this can be recognized by an alert user. In that case, the test elements so affected can be manually segregated. However, such visual recognition is often difficult and may require sufficient experience of the user to make it effective. Moreover, due to the methodical difficulties with doing this, such a visual check is rarely performed or is simply forgotten, especially by non-expert users.
In order to protect test elements from light, humidity, dirt, germs, dust, as well as mechanical deterioration, and/or to store them in sterile conditions, test elements are typically packaged, either individually or in a common package or storage container. However, packaging typically does not offer 100% protection. For example, test elements can be damaged during fabrication, transport or improper storage. Furthermore, typical packaging often does very little to protect the test elements against temperature influences or against the effects of aging. For test elements stored in common packaging or containers in particular, the aging process proceeds increasingly after the package or container has been opened the first time. Thus, consideration must be paid to the fact of the aging of test elements once a package or container is opened.
In view of the foregoing, it is important to verify that a test element is in a usable condition before its use. Certain known methods have been employed for this purpose. For example, in photometric measurement systems it is known to determine the so-called dry-blank value of the test field before applying the liquid sample to be analyzed. That is, the remission value of the test field on the test element is determined without a sample present. The dry-blank (or blank-field) value is compared to a pre-determined tolerance value or tolerance range, and the test element is rejected if the dry-blank value exceeds the value or is not within the range. In such a case, the dry-blank value for remission serves as a “control parameter”.
In this example, only a very inaccurate verification is possible because the measuring devices used for the evaluation of the test elements have their own unique tolerances during their manufacture, and because the differences between devices actually are amplified during their service life, e.g. due to changes in the device. This is particularly true of optical measurement devices. Changes can be caused by mechanical forces leading to mis-adjustment, aging of the illumination system (e.g. light emitting diodes), or dust deposits on the optical detection elements.
In addition to issues with the measurement devices, inaccuracy of known verification methods may also result from batch or lot fluctuations with respect to reagent and even test element fabrication, all of which influence the dry blank value. This can be overcome, however, by using batch- or lot-specific information or coding relating to the lest elements that is provided to the evaluation device, such as from the test elements themselves or from their packaging.
Other methods are known for correction or adjustment of analyses of samples provided on test elements in optical systems, such as U.S. Pat. No. 4,832,488 and DE 2800225 A1. Such methods, however, do not provide teachings relating to verifying usability of a test strip.
Taking into account the state of the art, an object of the present invention is to provide a simple and reliable method for verifying the usability of test elements in an analysis system. This object, and other objects that will be apparent from this disclosure to those of skill in the art, may be achieved by one or more of the embodiments disclosed and claimed herein for the present invention.